WHEN? HOW LONG? more popular uses. Time interval may or may not be associated with a specific date. A person timing the movement of a horse around a racetrack, for example, is concerned with the minutes, seconds, and fractions of a second between the moment the horse leaves the gate and the moment it crosses the finish line. The date is of interest only if he must have the horse at a particular track at a certain hour on a certain day. Time interval is of vital importance to synchronization, which means literally "timing together." Two military units that expect to be separated by several kilometers may wish to surprise the enemy by attacking at the same moment from opposite sides. So before parting, men from the two units synchronize their watches. Two persons who wish to communicate with each other may not be critically interested in the date of their communication, or even in how long their communication lasts. But unless their equipment is precisely synchronized, their messages will be garbled. Many sophisticated electronic communications systems, navigation systems, and proposed aircraft collision-avoidance systems have little concern with accurate dates; but they depend for their very existence on extremely accurate synchronization. The problem of synchronizing two or more time-measuring devices-getting them to measure time interval accurately and together, very precisely, to the thousandth or millionth of a second -presents a continuing challenge to electronic technology. Among the most fascinating remains of many ancient civilizations are their elaborate time-watching devices. Great stone structures like Stonehenge, in Southern England, and the 4,000-year-old passage grave of Newgrange, near Dublin, Ireland, that have challenged anthropologists and archaeologists for centuries, have proved to be observatories for watching the movement of heavenly bodies. Antedating writing within the culture, often by centuries, these crude clocks and calendars were developed by primitive peoples on all parts of our globe. Maya and Aztec cultures developed elaborate calendars in Central and North America. And even today scientists are finding new evidence that stones laid out in formation on our own western plains, such as the Medicine Wheel in northern Wyoming, formerly thought to have only a religious purpose, are actually large clocks. Of course they had religious significance, also, for the cycles of life-the rise and fall of the tides, and the coming and going of the seasons-powers that literally controlled the lives of primitive peoples as they do our own, naturally evoked a sense of mystery and inspired awe and worship. Astronomy and time so obviously beyond the influence or control of man, so obviously much older than anything the oldest man in the tribe could remember and as nearly "eternal" as anything the human mind can comprehend-were of great concern to ancient peoples everywhere. CLOCKS IN NATURE The movements of the sun, moon, and stars are easy to observe, and one can hardly escape being conscious of them. But of course there are countless other cycles and rhythms going on around us and inside of us-all the time. Biologists, botanists, and other life scientists study but do not yet fully understand many "built in" clocks that regulate basic life processes-from periods of animal gestation and ripening of grain to migrations of birds and fish; from the rhythms of heartbeats and breathing to those of the fertile periods of female animals. These scientists talk about "biological time," and have written whole books about it. Geologists also are aware of great cycles, each one covering thousands or millions of years; they speak and write in terms of "geologic time." Other scientists have identified quite accurately the rate of decay of atoms of various elements-such as carbon 14, for example. So they are able to tell with considerable dependability the age of anything that contains carbon 14. This includes everything that was once alive, such as a piece of wood that could have been a piece of Noah's Ark or the mummified body of a king or a pre-Columbian farmer. KEEPING TRACK OF THE SUN AND MOON Some of the stone structures of the earliest clock watchers were apparently planned for celebrating a single date-Midsummer Day, the day of the Summer solstice, when the time from sunrise to sunset is the longest. It occurs on June 21-22, depending on how near the year is to leap year. For thousands of years, the "clock" that consists of the earth and the sun was sufficient to regulate daily activities. Primitive peoples got up and began their work at sunrise and ceased work at sunset. They rested and ate their main meal about noon. They didn't need to know time any more accurately than this. But there were other dates and anniversaries of interest; and in many cultures calendars were developed on the basis of the revolutions of the sun, the moon, and the seasons. If we think of time in terms of cycles of regularly recurring events, then we see that timekeeping is basically a system of counting these cycles. The simplest and most obvious to start with is days sunrise to sunrise, or more usefully, noon to noon, since the "time" from noon to noon is, for most practical purposes, always the same, whereas the hour of sunrise varies much more with the season. One can count noon to noon with very simple equipment-a stick in the sand or an already existing post or tree, or even one's own shadow. When the shadow points due North-if one is in the northern hemisphere or when it is the shortest, the sun is at its zenith, and it is noon. By making marks of a permanent or semipermanent nature, or by laying out stones or other objects in a preplanned way, one can keep track of and count days. With slightly more sophisticated equipment, one can count full moonsor months-and the revolutions of the earth around the sun, or years. It would have been convenient if these cycles had been neatly divisible into one another, but they are not. It takes the earth about 36514 days to complete its cycle around the sun, and the moon circles the earth about 13 times in 364 days. This gave thorny problems to work out. THINKING BIG AND THINKING SMALL-AN ASIDE ON NUMBERS Some scientists, such as geologists and paleontologists, think of time in terms of thousands and millions of years. In their vernacular a hundred years more or less is insignificant-too small to recognize or to measure. To other scientists, such as engineers who design sophisticated communication systems and navigation systems, one or two seconds' variation in a year is intolerable because it causes them all sorts of problems. They think in terms of thousandths, millionths, and billionths of a second. The numbers they use to express these very small "bits" of time are very large. of a second, for example, is one 1 1,000,000 1 1,000,000,000 microsecond. of a second is one nanosecond. To keep 1 1,000,000' A billionth of a second is an almost inconceivably small bitmany thousands of times smaller than the smallest possible "bit" of length or mass that can be measured. We cannot think concretely about how small a nanosecond is; but to give some idea, the impulses that "trigger" the picture lines on the television screen come, just one at a time, at the rate of 15,750 per second. The whole picture "starts over," traveling left to right, one line at a time, the 525 lines on the picture tube, 30 times a second. At this rate it would take 63,000 nanoseconds just to trace out one line. Millionths and billionths of a second cannot, of course, be measured with a mechanical clock at all. But today's electronic devices can count them accurately and display the count in usable, meaningful terms. Whether one is counting hours or microseconds, the principle is essentially the same. It's simply a matter of dividing units to be counted into identical, manageable groups. And since time moves steadily in a "straight line" and in only one direction, counting the swings or ticks of the timer-the frequency with which they occur —is easier than counting the pellets in a pailful of buckshot, for example. "Bits" of time, whatever their size, follow one another single file, like beads on a string; and whether we're dealing with ten large bits-hours, for example-or 200 billion small bits, such 63 MICRO 525 LINES 30 TIMES/SEC TRACE MOVES BACK TO LEFT EDGE OF SCREEN TO START NEW LINE COUNTER 4 ANALA as microseconds, all we need to do is to count them as they pass through a "gate," and keep track of the count. The "hour" hand on a clock divides a day evenly into 12 or 24 hours-depending on how the clock face and works are designed. The "minute" hand divides the hour evenly into 60 minutes, and the "second" hand divides the minute evenly into 60 seconds. A "stop watch" has a finer divider-a hand that divides the seconds into tenths of a second. When we have large groups of identical items to count, we often find it faster and more convenient to count by tens, dozens, hundreds, or some other number. Using the same principle, electronic devices can count groups of ticks or oscillations from a frequency source, add them together, and display the results in whatever way one may wish. We may have a device, for example, that counts groups of 9,192,631,770 oscillations of a cesium-beam atomic frequency standard, and sends a special tick each time that number is reached; the result will be very precisely measured onesecond intervals between ticks. Or we may want to use much smaller bits-microseconds, perhaps. So we set our electronic divider to group counts into millionths of a second, and to display them on an oscilloscope. Electronic counters, dividers, and multipliers make it possible for scientists with the necessary equipment to "look at," and to put to hundreds of practical uses, very small bits of time, measured to an accuracy of one or two parts in 1011; this is about 1 second in 3,000 years. Days, years, and centuries are, after all, simply units of accumulated nanoseconds, microseconds, and seconds. |